1. Technical Field
The invention relates generally to dry type transformers having an iron core, a high voltage coil including a winding embedded in cast resin, and a low voltage coil including a winding embedded in cast resin, and also relates to a method of manufacturing the coils. The general structure of the transformer is similar to that described in Purohit et al U.S. Pat. No. 5,267,393, which is incorporated herein by reference.
2. Background Art
Dry type transformers with primary voltages over 600 volts have generally been constructed using one of three known techniques: conventional dry, resin encapsulated, or solid cast. The conventional dry method uses some form of vacuum impregnation with a solvent type varnish on a completed assembly consisting of the core and the coils of individual primary and secondary coils. Some simpler methods require just dipping the core and the coils in varnish without the benefit of a vacuum. This process inevitably results in voids or bubbles in the solidified varnish due to the presence of moisture and air, and thus does not lend itself to transformer applications above 600 volts. The resin encapsulated method encapsulates a winding with a resin with or without a vacuum, but does not use a mold to contain the resin during the curing process. This method does not insure complete impregnation of the windings with the resin and therefore the turn to turn insulation and layer insulation must provide the isolation for the voltage rating without consideration of the dielectric rating of the resin. The solid cast method utilizes a mold around the coil which is the principal difference between it and the resin encapsulated method. The windings are placed in the mold and impregnated and/or encapsulated with a resin under a vacuum, which is then allowed to cure before the mold is removed. Since all of the resin or other process material is retained during the curing process, there is a greater likelihood that the windings will be free of voids, unlike the resin encapsulated method whereby air can reenter the windings as the resin drains away before and during curing. Cooling channels can be formed as part of the mold.
Since the resin used in solid cast coils results in more of a solid bond between adjacent conductors than is possible with resin encapsulated coils, solid cast coils exhibit better short circuit strength among the windings. Part of this is because the conductors in the coils are braced throughout by virtue of the solid casting resin, and thus there is less likelihood of movement of the coils during short circuit conditions and short circuit forces are generally contained internally. An added benefit is that by having greater mass, there is a longer thermal time constant with the solid cast type coils and there is better protection against short term overloads.
In the field of high voltage transformers, the coils have been manufactured in the shape of cylinders to provide maximum short circuit strength. Specifically, under short circuit conditions, the outer windings of the coil have a natural tendency to expand in the form of a circle, and the inner windings of the coil have a natural tendency to constrict in the form of a circle. By forming the coil as a cylinder, the windings are already in the desired configuration that provides maximum short circuit strength. Moreover, from a mechanical strength standpoint, a cylindrical body will provide the highest strength as any induced loads will be distributed evenly around the entire circumference of the body.
A problem with the use of cylindrical coils, however, is that the transformer core must be manufactured so that the core fills as much of the inner space of the coil as possible (i.e., achieves a high core area/hole area ratio). Such cores typically are referred to as having a cruciform construction. This complex construction increases the overall expense of forming the transformer, and presupposes that the core material consists of fiat stacked laminations. Some newer core materials cannot practically be made into flat stacked laminations. Thus, the newer cores commonly consist of thin wound strips, most commonly of a single constant width, thus forming an essentially rectangular core cross-sectional shape.